ip directed-broadcast

A Smurf attack relies on the intermediary to broadcast ICMP echo request packets to hosts on a target subnet. When the ICMP echo request packet arrives at the target subnet, it is converted to a Layer 2 broadcast and sent to the connected hosts. This conversion takes place only when directed broadcast forwarding is enabled on the device. To avoid being an intermediary in a Smurf attack, make sure forwarding of directed broadcasts is disabled on the device.

The no form of the command disables directed broadcast forwarding.